Drag-to-reorder lists: manual sort_order is canonical, fractional re-slotting, haptic pickup/drop

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

app/src/lib/store.ts

@@ -114,7 +114,10 @@ export function createTask(data: Partial<Task> & { title: string }): Task {
     priority: data.priority ?? 4,
     estimate_min: data.estimate_min ?? null,
     completed_at: null,
-    sort_order: data.sort_order ?? 0,
+    // New tasks land at the top of their list.
+    sort_order:
+      data.sort_order ??
+      Math.min(0, ...state.tasks.filter((t) => t.status === (data.status ?? "next")).map((t) => t.sort_order)) - 1,
     created_at: now(),
     updated_at: now(),
   };
@@ -248,8 +251,27 @@ function byPriorityDue(a: Task, b: Task): number {
   );
 }
 
+// Manual arrangement wins: sort_order is the canonical list order (set by
+// drag-to-reorder); priority/due only break ties for rows that never moved.
+function byManualOrder(a: Task, b: Task): number {
+  return a.sort_order - b.sort_order || byPriorityDue(a, b);
+}
+
 export function tasksByStatus(s: State, status: Task["status"]): Task[] {
-  return s.tasks.filter((t) => t.status === status).sort(byPriorityDue);
+  return s.tasks.filter((t) => t.status === status).sort(byManualOrder);
+}
+
+/** Re-slot a task at `index` within its status list (fractional sort_order). */
+export function reorderTask(id: string, status: Task["status"], index: number) {
+  const list = tasksByStatus(getState(), status).filter((t) => t.id !== id);
+  const prev = list[index - 1]?.sort_order;
+  const next = list[index]?.sort_order;
+  const sort_order =
+    prev !== undefined && next !== undefined ? (prev + next) / 2
+    : prev !== undefined ? prev + 1
+    : next !== undefined ? next - 1
+    : 0;
+  updateTask(id, { sort_order });
 }
 
 /** Overdue + due-today + deferred-arriving-today, across all open lists. */